A. Field of the Invention
The present invention relates to a semiconductor device.
B. Description of the Related Art
As well as motor-controlling inverters, power devices are widely utilized in a large number of fields, such as large capacity PDPs (plasma display panels), power supply applications for FDPs (flat panel displays) such as liquid crystal panels, and inverters for domestic electrical appliances such as air conditioners or lighting. IGBTs (insulated gate bipolar transistors) and power MOSFETS (insulated gate field effect transistors) are known as this kind of power device.
To date, the drive and control of a power device has been carried out using an electronic circuit configured by combining a semiconductor device such as a photocoupler and an electronic part such as a transformer. However, owing to recent advances in LSI (large scale integrated circuit) technology, power devices are being used in 100 V AC (alternating current) and 200 V AC general household power supplies, 400 V AC industrial power supplies, and the like. Because of this, power devices with high voltage ICs of 100 V to 1,200 V breakdown voltage classes are being put to practical use.
High voltage ICs, being configured in various forms, are categorized into, for example, gate driver ICs incorporating a high-side gate driver and low-side gate driver of a power device, ICs incorporating an overheat protection or overcurrent protection function, inverter ICs wherein a control circuit and power device are integrated on the same semiconductor substrate (single-chip), and the like. Further, high voltage ICs contribute greatly to a reduction in size and increase in efficiency of an overall inverter system owing to a reduction in the number of parts mounted on a mounting substrate.
FIG. 16 is a circuit diagram showing a high voltage IC incorporating a general level shift circuit. In the circuit shown in FIG. 16, IGBTs (output power devices) 17 and 18 configure, for example, one phase of a PWM (Pulse Width Modulation) inverter bridge circuit. The IGBTs 17 and 18 are connected in series between a main direct current power supply (positive electrode side) Vdc supplying a high voltage of, for example, 400 V DC (direct current) or 400 V AC and a common potential COM (ground potential in FIG. 16), which is the negative electrode side of the main direct current power supply Vdc.
An OUT terminal, being a connection point of the emitter of IGBT 17 in the upper arm of the bridge circuit and the collector of IGBT 18 in the lower arm of the bridge circuit, is an alternating current output terminal of alternating current power generated by a complementary turning on and off of IGBT 17 and IGBT 18. An auxiliary direct current power supply (also called a driver power supply) E1 is such that the positive electrode is connected to a positive electrode line Vcc1, while the negative electrode is connected to the OUT terminal. An auxiliary direct current power supply (also called a driver power supply) E2 is such that the positive electrode is connected to a positive electrode line Vcc2, while the negative electrode is connected to the common potential COM. The auxiliary direct current power supplies E1 and E2 are low voltage power supplies of, for example, 15 V. Diodes 41 and 42 are connected respectively between the drain of a turn-on signal side high breakdown voltage MOSFET 1 and the alternating current output terminal OUT and between the drain of a turn-off signal side high breakdown voltage MOSFET 2 and the alternating current output terminal OUT, in order to cause surge current to flow to the common potential COM. Reference signs 51 and 52 are parasitic output capacities of the high breakdown voltage MOSFETs 1 and 2 respectively.
Furthermore, a level shift circuit and driver circuit 16 that drive IGBT 17 in the upper arm of the bridge circuit so as to be turned on and off, a driver circuit 20 that drives IGBT 18 in the lower arm of the bridge circuit so as to be turned on and off, and a control circuit (a low potential side low breakdown voltage circuit) 61 that inputs turn-on and turn-off signals into each of the driver circuits 16 and 20, are disposed in the high voltage IC shown in FIG. 16. The level shift circuit is configured of the high breakdown voltage MOSFETs 1 and 2, load resistors 3 and 4, NOT circuits 8 and 9 and subsequent stage low-pass filter circuits (hereafter referred to as LPFs) 30 and 31, an RS flip-flop (hereafter referred to as an RS latch) 15, and the like.
The level shift circuit and driver circuit 16 operate with the auxiliary direct current power supply E1 as a power supply. The driver circuit 20 operates with the auxiliary direct current power supply E2 as a power supply. The control circuit 61 is connected via the positive electrode line Vcc2 to the positive electrode of the auxiliary direct current power supply E2, and operates with the auxiliary direct current power supply E2 as a power supply. A high-side drive circuit (the circuit portion enclosed by the broken line in FIG. 16) 300 operates with the potential of the OUT terminal, which alternately follows the potentials of the common potential COM and main direct current power supply Vdc in accordance with a turning on and off of the IGBTs 17 and 18, as a reference. A bootstrap capacitor, for example, is used as the auxiliary direct current power supply E1 of the high-side drive circuit 300.
Control circuit 61 is connected to the gate of each of the high breakdown voltage MOSFETs 1 and 2, and to driver circuit 20. Control circuit 61 generates a set pulse turn-on signal 25 to be input into the gate of high breakdown voltage MOSFET 1, and reset pulse turn-off signal 26 to be input into the gate of high breakdown voltage MOSFET 2. Further, control circuit 61 inputs turn-on and turn-off signals into driver circuit 16 via the level shift circuit.
High breakdown voltage MOSFET 1 is energized by set pulse turn-on signal 25 input from control circuit 61. High breakdown voltage MOSFET 1 is a high breakdown voltage n-type channel MOSFET, and causes IGBT 17 to be turned on using the voltage drop of load resistor 3 connected to the collector of high breakdown voltage MOSFET 1 as a signal. High breakdown voltage MOSFET 2 is energized by the reset pulse turn-off signal 26 input from control circuit 61. High breakdown voltage MOSFET 2 is a high breakdown voltage n-type channel MOSFET, and causes IGBT 17 to be turned off using the voltage drop of the load resistor 4 connected to the collector of high breakdown voltage MOSFET 2 as a signal.
High breakdown voltage MOSFET 1 and high breakdown voltage MOSFET 2, and load resistor 3 and load resistor 4, are configured so as to be mutually equivalent in order that circuit constants coincide. Constant voltage diodes 5 and 6 connected in parallel to the load resistors 3 and 4 respectively restrict an excessive voltage drop of the load resistors 3 and 4, thereby protecting the NOT circuits 8 and 9 and the like. The two high breakdown voltage MOSFETs 1 and 2 are circuit portions of the level shift circuit that input signals based on the potential of the common potential COM into the level shift circuit.
End portions of load resistors 3 and 4 on the side opposite to the end portions on the side connected to high breakdown voltage MOSFETs 1 and 2 are connected to the positive electrode line Vcc1 to which the positive electrode of the auxiliary direct current power supply E1 is connected. Because of this, as the potential of the OUT terminal varies between the potential of the common potential COM and the main direct current power supply Vdc potential, the power supply voltage of a load resistor circuit of high breakdown voltage MOSFETs 1 and 2 formed of load resistors 3 and 4 varies between a power supply voltage wherein the voltages of the auxiliary direct current power supply E1 and main direct current power supply Vdc are added together and the power supply voltage of the auxiliary direct current power supply E1.
A method whereby input signals input into IGBT 17 in the upper arm of the bridge circuit are controlled by the two high breakdown voltage MOSFETs 1 and 2 in this way is normally called a two-input method. When configuring the level shift circuit using the two-input method, the output of the OUT terminal becomes HIGH when set pulse turn-on signal 25 is input into high breakdown voltage MOSFET 1. Because of this, the potential of the OUT terminal rises from the potential of the common potential COM to the main direct current power supply Vdc potential.
Actually, however, the potential of the OUT terminal transiently jumps higher than the main direct current power supply Vdc potential due to an inductance element caused by a load such as a motor, or wiring or the like, connected to the OUT terminal. Because of this, in order to avoid destruction due to this switching noise, it is necessary to guarantee a breakdown voltage for a high voltage IC and power devices such as the high breakdown voltage MOSFETs 1 and 2 such that a voltage higher than the main power supply voltage on the high voltage side can be withstood. For example, commercially available high voltage ICs and power devices are such that a breakdown voltage of 600 V is guaranteed in the case of a 200 V AC system power supply, and a breakdown voltage of 1,200 V is guaranteed in the case of a 400 V AC system power supply.
FIG. 17 is a plan view showing a planar structure of a heretofore known high voltage IC. As shown in FIG. 17, a high voltage IC 1000 is formed in an n-type diffusion (or n-type epitaxial) region 1001 provided on a p-type substrate (not shown) having ground potential. In order to realize a high breakdown voltage of 600 V or 1,200 V, a high breakdown voltage junction termination structure (HVJT) region 1011 and a high breakdown voltage device such as a high breakdown voltage n-type channel MOSFET 1012 are incorporated in high voltage IC 1000. Depending on the protective function form, a high breakdown voltage device such as high breakdown voltage p-type channel MOSFET 1013 is also incorporated.
High breakdown voltage junction termination structure region 1011 encloses a high potential region in which high-side drive circuit 300 is provided. High breakdown voltage n-type channel MOSFET 1012 and high breakdown voltage p-type channel MOSFET 1013 are provided in high breakdown voltage junction termination structure region 1011. The high breakdown voltage n-type channel MOSFET 1012 is connected to control circuit 61 and high-side drive circuit 300, and configures, for example, a level shift circuit. High breakdown voltage p-type channel MOSFET 1013 is connected to control circuit 61. Each of high breakdown voltage junction termination structure region 1011, high breakdown voltage n-type channel MOSFET 1012, and high breakdown voltage p-type channel MOSFET 1013 realizes a desired breakdown voltage in a junction portion between the p-type substrate and n-type diffusion region 1001, to which a high voltage is applied.
Next, a description will be given of the reliability of a high voltage IC including a withstand region. A withstand region is a region in which high breakdown voltage junction termination structure region 1011, a high breakdown voltage device such as high breakdown voltage n-type channel MOSFET 1012, and an element separation region are provided. When the high breakdown voltage device configuring the withstand region is of, for example, a lateral device structure, the electric field on the withstand region surface between the anode and cathode, or between the source and drain (between a high voltage electrode and a low voltage electrode), of the high breakdown voltage device increases more the higher the voltage applied to the high voltage IC. Because of this, the breakdown voltage of the high voltage IC drops or fluctuates due to movable ions or a charge accumulation in the mold resin, because of which the reliability of the high voltage IC decreases.
A device in which is provided a capacitive field plate structure wherein polysilicon or metal is capacitively coupled to a field plate electrode provided across a dielectric on the withstand region surface has been proposed as a device that eliminates this kind of problem (for example, refer to JP-A-2002-353448 and Japanese Patent No. 3,591,301). Also, a device in which is provided a resistive field plate structure wherein an oxygen-doped semi-insulating polysilicon (SIPOS) thin film or a high resistance polysilicon thin film is disposed in a spiral form across a dielectric on the withstand region surface from another region (hereafter referred to as a low potential region) separated from a high potential region by a surface element separation region to the high potential region has been proposed as another device (for example, refer to Japanese Patent No. 3,117,023, U.S. Pat. No. 7,183,626 and JP-A-2003-8009).
However, when the breakdown voltage guaranteed in the withstand region is extremely high, such as in a high voltage IC of, for example, a 1,200 V breakdown voltage class, it is necessary that the impurity concentration of the withstand region be extremely low. However, due to the impurity concentration of the withstand region surface being low, and the voltage applied to the high voltage IC being high, the adverse effect on the breakdown voltage characteristics of the high voltage IC caused by movable ions or charge of the mold resin is even more noticeable.
In this case, the movable ions or charge of the mold resin accumulate on a protective film of the high voltage IC, because of which, with a capacitive field plate structure, it is not possible to maintain an even potential distribution in the withstand region with respect to the amount of movable ions or charge accumulated on the protective film on the withstand region. Because of this, when the breakdown voltage guaranteed in the withstand region is extremely high, or when the amount of charge contained in the mold resin is extremely large, it is often the case that the resistive field plate structure is applied such that deterioration in breakdown voltage characteristics due to movable ions or charge in the mold resin is unlikely to occur.
A resistive field plate structure is such that when, for example, a high voltage is applied between the high voltage electrode and low voltage electrode of the withstand region, a minute current flows in the resistive field plate structure, and a successive voltage drop occurs from the high potential region side to the low potential region side. Because of this, the potential distribution in the withstand region is forcibly kept even, and the electric field on the withstand region surface is alleviated, because of which stable breakdown voltage characteristics are obtained. A device wherein a resistive field plate structure is actually disposed in a spiral form on the withstand region of a high voltage IC of a 1,200 V breakdown voltage class has also been proposed (for example, refer to M. Yoshino et al., “A new 1200 V HVIC with a novel high voltage Pch-MOS”, Proceedings of the 22nd International Symposium on Power Semiconductor Devices & ICs, 2010, pages 93 to 96.).
Next, a description will be given of a planar structure of a resistive field plate structure shown in Yoshino et al. (2010). FIG. 18 is a plan view showing a planar structure of a heretofore known resistive field plate structure. Also, FIG. 19 is a characteristic diagram showing breakdown voltage characteristics of a heretofore known high breakdown voltage device. FIGS. 18 and 19 are FIGS. 2 and 12 respectively of Yoshino et al. (2010). As shown in FIG. 18, high breakdown voltage junction termination structure region 1203 including a high breakdown voltage n-type channel MOSFET or high breakdown voltage p-type channel MOSFET (not shown) is provided between high potential region 1201 and low potential region 1202. High resistance polysilicon thin film 1204 configuring a resistive field plate structure is disposed in a spiral form on high breakdown voltage junction termination structure region 1203.
The leakage current when a high voltage of 1,200 V is applied to a high-side high voltage terminal (not shown) in the high breakdown voltage junction termination structure region 1203 is approximately 30 μA. Normally, a high breakdown voltage p-type channel MOSFET is used as a level-down device that transmits an error signal from a high-side drive circuit portion to a low-side control circuit in order to detect an overcurrent in the upper arm IGBT. Because of this, the source of the high breakdown voltage p-type channel MOSFET is connected to the positive electrode line Vcc1 to which is connected the positive electrode of the auxiliary direct current power supply E1, which is the power supply voltage of the high-side drive circuit portion.
Consequently, the occurrence of a resistive leak as shown in FIG. 19 indicates that one end portion of the high resistance polysilicon thin film 1204 configuring the resistive field plate structure is connected to the positive electrode line Vcc1. That is, the leakage current in Yoshino et al. (2010) is a leak element flowing through the high resistance polysilicon thin film connected between the positive electrode line Vcc1 and the ground.
A device wherein an alleviation of the withstand region surface electric field is attempted by providing a combination of a capacitive field plate structure and a resistive field plate structure has been proposed as a device that reduces the adverse effect of the leakage current on a high voltage IC, and reduces the adverse effect of movable ions or a charge on the high voltage IC (for example, refer to JP-A-2005-5443).
Generally, with a power device gate drive method using a high voltage IC (HVIC), a bootstrap circuit formed of a bootstrap diode (BSD) and bootstrap capacitor is connected to high-side drive circuit 300 shown in FIG. 16. In this case, the voltage of the bootstrap capacitor configuring the auxiliary direct current power supply E1 (hereafter referred to as the bootstrap capacitor E1) becomes the power supply voltage of high-side drive circuit 300, and also becomes the gate voltage of upper arm IGBT 17. The bootstrap capacitor E1 is a floating power supply whose power supply voltage varies in accordance with the potential of the OUT terminal.
A description will be given of a charge and discharge cycle of the bootstrap capacitor E1. As a basic operation of high-side drive circuit 300, when an output signal of a low-side drive circuit (low-side driver) formed of driver circuit 20 is at a high level, lower arm IGBT 18 changes to an on-state, and the potential of the OUT terminal is pulled down to the common potential COM. For this period, the bootstrap capacitor E1 is charged by the forward current of the BSD whose anode electrode is connected to the positive electrode line Vcc2 to which the positive electrode of the auxiliary direct current power supply E2 is connected. The voltage of the bootstrap capacitor E1 is charged to, for example, 14.4 V, which is the value obtained after the BSD forward voltage drop (VF) 0.6 V is subtracted from 15 V.
Meanwhile, when the output signal of the low-side drive circuit is at a low level, lower arm IGBT 18 changes to an off-state. In a subsequent dead time period, upper arm IGBT 17 changes to an on-state when an output signal of the high-side drive circuit (high-side driver) 300 is at a high level, and the potential of the OUT terminal rises to the voltage of the main direct current power supply Vdc (and transiently to a still higher voltage). For this period, the charge of the bootstrap capacitor E1 is released in order to charge the gate capacitor of upper arm IGBT 17. The amount of the bootstrap capacitor E1 voltage drop caused by the discharge may be approximately several volts, depending on upper arm IGBT 17 gate capacitance and gate-to-source leakage current, and on the amount of leakage current from the positive electrode line Vcc1 to the ground.
When the voltage of the bootstrap capacitor E1 drops, the drive capability of upper arm IGBT 17 decreases, and the output current of IGBT 17 decreases. Also, when the voltage of the bootstrap capacitor E1 drops beyond the stopping voltage of a UVLO (Under Voltage Lock Out, not shown) provided in high-side drive circuit 300, there is concern that the output of high-side drive circuit 300 will stop. For example, in the event that the voltage of the bootstrap capacitor E1 drops from 14.4 V to 10.4 V due to a discharge when the stopping voltage of the UVLO function is 11 V, the output of high-side drive circuit 300 will stop.
Normally, the UVLO is a high-side CMOS logic circuit provided between the RS latch 15 (R terminal) and positive electrode line Vcc1 in high-side drive circuit 300. Consequently, when designing high-side drive circuit 300, design is carried out so that the voltage of the bootstrap capacitor E1 is charged to a desired voltage (herein, 14.4 V), and the drive capability of upper arm IGBT 17 is always fulfilled to the maximum, taking into consideration parameters such as a gate capacitance Qg of the IGBTs 17 and 18, the capacitance of the bootstrap capacitor E1, the backflow prevention performance of the bootstrap diode, the amount of leakage current from the positive electrode line Vcc1 to the ground, and the on-state time of upper and lower arm IGBTs 17 and 18.
However, in the case of disposing a resistive field plate structure on the withstand region in order to achieve increased reliability when a breakdown voltage class of 1,200 V or the like is adopted, or when the amount of charge contained in the mold resin is large, the high resistance polysilicon thin film is connected to each of the positive electrode line Vcc1 and the ground in order to maintain breakdown voltage characteristics and for ease of layout, because of which the amount of leakage current from the positive electrode line Vcc1 to the ground increases.
For example, normally, when providing a capacitive field plate structure on the withstand region, the amount of leakage current from the positive electrode line Vcc1 to the ground is approximately several hundred nA to several μA, but when providing a resistive field plate structure on the withstand region, the amount of leakage current from the positive electrode line Vcc1 to the ground is larger than when providing a capacitive field plate structure. A specific calculation will be made of the amount of leakage current from the positive electrode line Vcc1 to the ground in the case of providing a resistive field plate structure on the withstand region.
For example, when taking the sheet resistance value of the high resistance polysilicon thin film to be 2 kΩ/sq, the peripheral length of the high breakdown voltage junction termination region to be 1 mm, the high resistance polysilicon thin film to be disposed in a spiral form to a width of 1 μm and at 1 μm intervals, and the width of the high breakdown voltage junction termination region from the high potential region to the element separation region (hereafter referred to as the width of the high breakdown voltage junction termination region) to be 150 μm, the high resistance polysilicon thin film is disposed to approximately 15 spirals on the high breakdown voltage junction termination region. The width of the high resistance polysilicon thin film is the width of the high resistance polysilicon thin film in a direction perpendicular to the direction in which the spirals of the high resistance polysilicon thin film extend. The interval of the high resistance polysilicon thin film is the interval between spiral lines adjacent in the width direction of the high resistance polysilicon thin film (hereafter referred to as the spiral interval). A total resistance value Rpoly of the high resistance polysilicon thin film at this time is calculated as in Equation (1) below.Rpoly=2,000×(1,000/1)×15=30,000,000(Ω)  (1)
Also, when supposing that the potential of the positive electrode line Vcc1 rises to 1,200 V, a leakage current Ileak from the positive electrode line Vcc1 to the ground is calculated as in Equation (2) below.Ileak=1,200/30,000,000=40×10−6(A)  (2)
As shown in Equations (1) and (2) above, the leakage current of a high voltage IC when providing a resistive field plate structure on the high breakdown voltage junction termination region is as much as 40 μA greater than when providing a capacitive field plate structure on the high breakdown voltage junction termination region. In this way, the leakage current of a high voltage IC when providing a resistive field plate structure on the high breakdown voltage junction termination region is as much as 10 times or more greater than when not providing a resistive field plate structure.
FIG. 15 is an illustration showing discharge paths of the bootstrap capacitor of the heretofore known high voltage IC. High voltage IC 1000 shown in FIG. 15 is such that the discharge paths of the bootstrap capacitor E1 are indicated in the high voltage IC circuit diagram shown in FIG. 16. As shown in FIG. 15, when upper arm IGBT 17 is turned on, the bootstrap capacitor E1 is discharged along the following first to third paths 71 to 73. The first path 71 (indicated by a dotted line arrow) is the path of a current flowing when applying a gate voltage to upper arm IGBT 17 via the positive electrode line Vcc1 and high-side drive circuit 300.
The second path 72 (indicated by a coarser arrow than that of reference sign 71) is the path of the reverse leakage current of a high breakdown voltage diode 1400 configuring the high breakdown voltage junction termination region connected between the positive electrode line Vcc1 of high voltage IC 1000 and the ground (the common potential COM). The paths of the reverse leakage current of a level shifter element (the constant voltage diodes 5 and 6 of FIG. 16) 1402 connected in parallel via a level shift resistor (the load resistors 3 and 4 of FIG. 16) to the high breakdown voltage diode 1400, and of the body diodes of the high breakdown voltage MOSFETs 1 and 2, are also included in the second path 72.
The third path 73 (indicated by a finer arrow than that of reference sign 71) is the path of the leakage current in resistive field plate structure portion 1401 connected in parallel to high breakdown voltage diode 1400. The second path 72 and third path 73 are paths in the high voltage IC along which the bootstrap capacitor E1 is discharged. The current discharged along the second path 72 is approximately several hundred nA to several μA, and can be ignored as it is sufficiently small with respect to the current of approximately 60 μA discharged along the third path 73.
Next, a description will be given of the amount by which the voltage of the bootstrap capacitor E1 drops due to the discharge along the third path 73 (hereafter referred to as a voltage drop amount). A voltage drop amount ΔVbs (V) of the bootstrap capacitor E1 due to the discharge is represented by Equation (3) below. In Equation (3), the capacitance of the bootstrap capacitor E1 is taken to be Cbs(F), the leakage current from the positive electrode line Vcc1 to the ground to be Ileak (A), and upper arm IGBT 17 to be in an on-state from t1(S) to t2(s).Math. 1ΔVbs={∫t1t2Ileak·dt}/Cbs  (3)
In order to reduce the discharge drop amount ΔVbs of the bootstrap capacitor E1, it is conceivable judging from Equation (1) to increase the capacitance of the bootstrap capacitor E1 (for example, by using a large capacity electrolytic capacitor, or the like), or to reduce the period for which upper arm IGBT 17 is in an on-state (=t2−t1, hereafter referred to as the on-state period). However, a problem occurs in that the PCB (Printed Circuit Board) area of an inverter power supply system will be increased by increasing the capacity of an electrolytic capacitor (approximately several μF). Also, by reducing the on-state period of upper arm IGBT 17, the standard width between the minimum on-state time t1 and maximum on-state time t2 narrows, and a problem occurs in that the operation of the high voltage IC is restricted.
Also, in order to reduce the discharge drop amount ΔVbs of the bootstrap capacitor E1, it is conceivable judging from Equation (1) to further increase the total resistance value of resistive field plate structure portion 1401, thereby reducing the leakage current amount Ileak from the positive electrode line Vcc1 to the ground. In order to reduce the leakage current amount Ileak, it is necessary to reduce the width and spiral interval of a high resistance polysilicon thin film having a spiral planar form, thereby reducing the disposition area of the high resistance polysilicon thin film. However, when disposing the high resistance polysilicon thin film with the heretofore described dimensions (1 μm width and 1 μm interval) or lower, the following kinds of problem occur.
For example, when reducing the width of the high resistance polysilicon thin film, problems occur with processing accuracy in the manufacturing process in that it is difficult to optimize resist exposure conditions when patterning the high resistance polysilicon thin film, the high resistance polysilicon thin film becomes detached when etching the high resistance polysilicon thin film, and the like. Also, when reducing the spiral interval of the high resistance polysilicon thin film far too much, polymers, particles, and the like, adhere to the side surface or upper portion of the high resistance polysilicon thin film. Because of this, a short circuit may occur within the high resistance polysilicon thin film. Because of this, realistically, it is also difficult to reduce the disposition area of the high resistance polysilicon thin film.
Also, in order to reduce the leakage current amount Ileak, it is conceivable to increase the width of the withstand region of the high breakdown voltage junction termination region in resistive field plate structure portion 1401 to, for example, approximately 300 μm, thereby increasing the number of spirals of the high resistance polysilicon thin film, and increasing the total resistance value of resistive field plate structure portion 1401. However, it is necessary to optimize the high breakdown voltage device structure in order to increase the width of the withstand region of the high breakdown voltage junction termination region, and the chip area also increases, which is not desirable as the cost increases considerably.
Also, it is conceivable to increase the sheet resistance value of the high resistance polysilicon thin film itself in order to increase the total resistance value of resistive field plate structure portion 1401. For example, when forming a high resistance polysilicon thin film doped with an impurity to a low impurity concentration on resistive field plate structure portion 1401 so that the sheet resistance value is approximately 10 kΩ/sq, the high resistance polysilicon thin film provided on high-side drive circuit 300, low-side drive circuit, and control circuit 61 in the high voltage IC is also formed with a sheet resistance value of approximately 10 kΩ/sq, in the same way as the high resistance polysilicon thin film of resistive field plate structure portion 1401.
Because of this, when the sheet resistance value of the high resistance polysilicon thin film other than that on resistive field plate structure portion 1401 is approximately 10 kΩ/sq, fluctuation in the resistance division ratio and absolute resistance value increases, and the accuracy of the resistor element itself in the high voltage IC deteriorates. Because of this, it is necessary to form a high resistance polysilicon thin film exclusively for increasing the resistance of resistive field plate structure portion 1401, separate from the high resistance polysilicon thin film on high-side drive circuit 300, low-side drive circuit, and control circuit 61 in the high voltage IC. Consequently, it is necessary to add a photolithography step, an ion implantation step, and the like, exclusively for forming a low impurity concentration high resistance polysilicon thin film on resistive field plate structure portion 1401, and there is a problem in that the manufacturing cost increases.